Optical disk players capable of reading information stored on DVD (Digital Video Disc) and other optical recording media are examples of optical recording/reproducing devices. Although 4.7-GB DVDs have been recently introduced to the market, strong demand still exists for higher density package media (next generation high density optical disk). A lot of effort has been dedicated to address the need. It is well known that effective ways to enhance recording density are reducing the wavelength of the light source and increasing the NA of the objective lens. Current standards for next generation high density optical disks are an objective lens with a numerical aperture (NA) of 0.85 and a light source at a wavelength of 405 nm, which are improvements over 0.6 and 650 nm respectively for conventional DVDs. The specifications are targeting a smaller focused spot, hence a higher recording density.
Increasing the NA of an objective lens leads to quick increases in coma aberration, a phenomenon which occurs when the optical disk is tilted. This in turn raises an issue of poor quality light convergence to the focused spot. The coma aberration caused by a tilt of the optical disk is proportional to the thickness of the optical transmissive layer from the light entering plane to the information recording plane. Accordingly, increases in coma aberration caused by an increasing NA can be restrained by reducing the thickness of the optical transmissive layer of the optical disk. This approach forms the basis of a current proposal to reduce the thickness of the optical transmissive layer of the next generation high density optical disk from a conventional DVD's 0.6 mm to 0.1 mm.
Incidentally, the next generation high density optical disk is required to be compatible with DVDs. Stated differently, the recording/reproducing device for the next generation high density optical disk is naturally required to be capable of recording/reproducing data on DVDs which are now in widespread use.
However, as described above, the next generation high density optical disk differs from the DVD in thickness of the optical disk substrate, making it difficult to ensure compatibility. An ordinary objective lens is designed assuming a substrate thickness of a particular optical disk; if the lens is used with an optical disk having a vastly different substrate thickness from the designed value, a spherical aberration, hence poor light convergence occurs at the focused spot.
Methods of solving such problems in the recording/reproduction on the optical disk which are caused by a difference in substrate thickness are disclosed in Published Unexamined Patent Application 8-55363 (Tokukaihei 8-55363/1996, published on Feb. 27, 1996; “the first conventional example”) and Proceedings No. 3, Page 1084, for Lecture No. 28aF-7 in 43th Conference of Applied Physics Society (the Proceedings distributed and Conference held on Mar. 26, 1996; “the second conventional example”). The prior art documents describe a method of rendering a DVD having a substrate thickness of 0.6 mm for use with a wavelength of about 650 nm and a CD (Compact Disc) having a substrate thickness of 1.2 mm for use with a wavelength of about 780 nm compatible with each other.
The first conventional example employs two light sources with wavelengths of 635 nm and 780 nm. The 635-nm light source is located at the focal point of the collimating lens to shine parallel light. The 780-nm light source is however located a little displaced from the focal point of the collimating lens, so as to shine somewhat diverging light. The arrangement of the lenses enables correction of the spherical aberration caused by different substrate thicknesses. The second conventional example employs two light sources with wavelengths 635 nm of 785 nm. In the path of the light flux hitting the objective lens is provided a diffractive element effective only to a light flux at a wavelength of 785 nm. The lens/grating device combination enables correction of the spherical aberration caused by different substrate thicknesses.
However, it entails the following problems to apply the conventional techniques to make the next generation high density optical disk compatible with the DVD.
The first conventional example, when applied to the compatibility issue between the DVD and the CD where the two wavelengths employed have a small difference and the objective lens has a small numerical aperture, reduces spherical aberration to some extent. However, when applied to the compatibility issue between the next generation high density optical disk and the DVD where the two wavelengths has a large difference and the objective lens has a high numerical aperture, the technique fails to correct aberrations which occurs to divergent incident light and poorly converges a diverging flux of incident light to the focused spot. A method to prevent this from happening is to provide a correction lens in the path of diverging light, where the correction lens must be placed only in that path of diverging light so that correction is effective only for diverging light, not for parallel light. The correction lens cannot therefore be driven integrally with the objective lens, making it difficult to maintain performance when the objective lens is moved.
The second conventional example, employing a diffractive element for correction purposes, is able to utilize light only with reduced efficiency, because the light used to address the compatibility issue between the next generation high density optical disk and the DVD come in two wavelengths: 405-nm blue light and 650-nm red light. The technique therefore requires a laser source with a high output power, which raises a problem of high power consumption. Such a low efficiency technique is not suited to an application in an optical pickup, also in view of the need for high output power in recording/erasing information on an optical disk.